The invention relates to a magneto-optical device having a substrate on which a magneto-optical layer is arranged. The magneto-optical layer is a metallic ferromagnetic material.
When a linearly polarized light beam is transmitted or reflected by the magneto-optical layer, the polarization of the beam changes as a function of the magnetization of the layer at the location where the light beam impinges on the layer. The change of the polarization may be a function of the ellipticity or of the position of the plane of polarization.
For example, when the position of the plane of polarization varies, dextrorotation or levorotation thereof takes place, depending on whether the magnetization responsible for the rotation influences the light beam with a positive or negative polarity. When an analyzer is placed in the light path of a light beam reflected or transmitted by a magneto-optical layer, the light beam will have different intensities depending on the magnitude of the rotation of the plane of polarization.
The above-mentioned magneto-optical effects may be used in a number of different magneto-optical devices. For example, magnetic data can be scanned by means of a focused light beam. The light beam is reflected by the recording medium at the area of the magnetic recordings. The differences in intensity of the reflected light beam detected by means of an analyzer represent the written magnetic data. All this may be carried out, for example, in such a manner that the analyzer placed in the light path transmits a light beam of maximum intensity when a location having a magnetization of one polarity is scanned, and that the analyzer transmits a light beam of minimum intensity when a location having an equally large magnetization but opposite polarity is scanned. In this manner data recorded magnetically can be read optically.
Another application is as a magneto-optical mirror in ring laser gyroscopes.
The material which, at room temperature, exhibits the largest known magneto-optical effect with reflected light (the Kerr effect) is MnBi. However, crystallographically this material is not stable. This means that upon heating MnBi to temperatures in the proximity of its Curie temperature, the MnBi crystal structure transforms to a phase having a considerably smaller magneto-optical effect. As a result of this, MnBi is not suitable for repeated thermomagnetic recording of magnetic data. This is because in a thermomagnetic recording process the material should be locally heated to a temperature in the proximity of its Curie temperature so as to be able to reverse the direction of magnetization.
Other metallic magneto-optical materials, for example, GdCo and GdFe are crystallographically stable but exhibit considerably smaller magneto-optical effects than MnBi.
In this connection it is to be noted that certain oxidic materials (ferrites and garnets) have a very high magneto-optical Q-factor for transmitted light (defined as the Faraday effect divided by the absorption coefficient). However, this property of oxidic materials is associated with a low absorption. As a result, these materials must have rather large thicknesses (approximately 0.5 microns) when used in recording devices. This means that writing is difficult (writing requires much electric power), and that in fact very small bits cannot be written (the recording density is too low).
Magneto-optic metallic materials can be used in reflection, for which thin layers (having thickness smaller than 0.5 microns, in particular between 0.01 and 0.2 microns) are suitable. Recording information in these thin layers requires little electric power (the absorption of metal layers is high) and writing very small bits has proved to be possible. Moreover, thin metal films are cheap to make as compared with, for example, monocrystalline garnet films. All these factors cause the interest in metal films for magneto-optical applications to be great.